1. Field of the Invention
The invention relates to a process for vitrifying heavy-metal-containing residues having a chlorine content above 10% by mass from flue gas cleaning, in particular filter dust and residues from flue gas scrubbing. In the process the residues are melted in a melting furnace and are then discharged, with fluxes being able to be added to condition the melt. The invention also relates to a melting furnace for carrying out the process. A process of this generic type is disclosed by DE 43 40 754 A1, for example, and a Deglor furnace is described, for example, in EP 0 633 411 A1.
2. Discussion of Background
It is known to treat heavy-metal-containing residues, such as filter ash from the incineration of refuse, by melt processes. At temperatures of approximately from 1300 to 1400.degree. C., these residues are generally melted without fluxes in an electrical melting furnace, discharged from the furnace and then cooled, so that a vitreous residue is formed which can be landfilled without problem.
In the Deglor process, heating above the melt as well ensures that, on account of sufficiently high temperatures in the gas space, a considerable proportion of the heavy metals is evaporated. The exhaust gas taken off from the furnace by a fan is cooled and passed through a bag filter, in which the heavy metals which have condensed out are collected as condensate. This condensate can be treated in a metal works to recover the heavy metals. The molten residues are discharged from the furnace via a gas-tight siphon and then cooled, in which case a vitreous product is formed. The heavy metal evaporation contributes to the fact that the quality of the glass product permits reusability. This makes up a considerable advantage of the Deglor process in comparison with other known melting processes in which, because of the lack of heating in the upper furnace, there is no evaporation of the heavy metals.
The Deglor process has proved most useful in the vitrification of residues having a comparatively low content of metal chlorides, metal sulfates and metal sulfites, as are typically present in filter dust.
However, residues also arise in which the filter ash is admixed with the flue gas cleaning product which has a high chlorine content. Thus, to remove sulfur dioxide and hydrochloric acid, the exhaust gases are subjected to a lime scrubbing, calcium compounds, in particular CaSO.sub.3, CaSO.sub.4 and CaCl.sub.2, arising as byproducts which are to be melted and vitrified together with the filter ash. Typically, this product comprises from 10 to 20% by mass of chlorine, predominantly in the form of CaCl.sub.2. This compound has a boiling point (&gt;1600.degree. C.) far above the customary operating temperatures. This means that these chlorides cannot be transferred to the vapor phase effectively enough. At the same time, only a small portion of chlorine can be incorporated into the glass.
In order to be able to melt this residue with a high salt content likewise in an environmentally compatible manner, the applicant has suggested a process in which an alkaline flux in the form of an oxide, hydroxide or carbonate is added to the residues, after mixing the stoichiometric ratio between the sum of the alkalines and the chlorine being greater than 0.75, preferably greater than 1 (DE 196 03 365.9). The admixture of suitable alkali-metal-containing additives and the assurance of a sufficiently high exhaust gas volume significantly increases the evaporation rate of the chlorides.
Since the chlorides predominantly evaporate as NaCl, disadvantageously, a considerably greater amount of condensate is produced in which the heavy metals are then present in a form so dilute that treatment is no longer worthwhile.
A further disadvantage is that on account of the additives additionally required, the additional rate of exhaust gas and the greater quantity of material to be evaporated off, the energy requirement of the Deglor unit increases.
The two last-mentioned disadvantages are also present if, instead of lime, alkali-metal-containing reagents, such as NaHCO.sub.3, for example, are used for the flue gas cleaning, as a result of which the addition of alkali-metal-containing fluxes to the melting furnace is no longer necessary to increase the evaporation rate.